A base station in a cellular network is assigned a unique identifier, known in LTE as the enhanced Cell Global Identifier (eCGI), and a further ID known as the Physical Cell Identifier (PCI). There are only 504 PCIs available for all base stations to share, which can lead to issues collectively known as PCI conflicts. A first type of PCI conflict is a PCI collision, which is shown in FIG. 1a. This occurs when two neighboring base stations have the same PCI and have overlapping coverage areas. As the first base station's coverage area covers the second base station (as shown in FIG. 1a), then the second base station can detect the first base station's PCI via a Radio Environment Monitoring (REM) scan, detect the PCI collision, and initiate a PCI conflict resolution process. However, if the two base stations respective coverage areas do not cover the other base station, then this is not possible. This can lead to issues as a User Equipment (UE) in the overlapping area cannot differentiate between signals from either base station. This leads to significant interference and can make channel estimation extremely difficult.
The other type of PCI conflict is PCI confusion, an example of which is shown in FIG. 1b. In this case, a first base station has a coverage area that overlaps with the coverage areas of a second and third base station, but the second and third base station's coverage areas do not overlap and no coverage area covers a neighboring base station. The second and third base station have the same PCI. In this case, no base station can detect the PCI of a neighbor as no base station receives signals from a neighboring base station. Instead, the first base station will receive measurement reports from UEs in both the overlapping coverage area of the first and second base station and the overlapping coverage area of the first and third base station. However, these measurement reports include both the second and third base stations with the same PCI. This may cause handover issues as the first base station cannot differentiate between these two neighboring base stations in a handover process (without additional control signaling to determine further identifiers for the other base stations).
More recent cellular telecommunications networks are known as heterogeneous networks (or “HetNets”) as they comprise a mixture of the conventional macro base station and other types of base stations such as the femtocell. A femtocell, also known as a Home evolved Node B (HeNB) is one of a class of base stations known as small cells, which further includes picocells, metrocells and microcells depending on the coverage area. It is intended to deploy HeNBs such that there is a much higher density of HeNBs compared to macro base stations. This has the advantages of increased coverage and capacity for the cellular network. However, the limited number of PCI values and the likely self-organised nature of the femtocell means that such a deployment will likely result in more PCI conflicts, which would reduce the positive impact of the technology.